Topics
Analog and Digital signals
Telephone lines
Cables
Development of cables
The Atlantic telegraph cable
The spread of telephone cables
Recent development
Types of cables
Coaxial cables
Fiber optics
Definitions
voice, analog signals, digital signals
amplitude, binary digits, modulation
cables, demodulation, an amplifier
susceptible, electric current, to transmit
an insulation, hemp, a cable
to strengthen, a message, voice
subscribing, an interference, sheath
a conductor, demodulation, a cable
analog signals, digital signals, binary digits
a sheath, a conductor, a modulation
Analog and Digital Signals
Words and word combinations:
Continuous – непрерывный
a steady stream of water – постоянный поток воды
a garden hose – садовый шланг
is measured – измеряется
cycles per second – циклы в секунду
high-frequency sound waves – высокочастотные звуковые волны
low-frequency waves – низкочастотные волны
loudness – громкость
the short bursts of water – брызги воды
garden sprinkler – садовый разбрызгиватель
transmission medium – средства передачи
electrical pulses – электрические пульсы
digital data – цифровые данные
converting digital signals into analog – превращение цифровых сигналов в аналоговые
converting the analog signals back into digital – превращение аналоговых сигналов в цифровые
sending computer – отправляющий компьютер
receiving computer – принимающий компьютер
must be connected to a modem – должны быть соединены с модемом
both modems – оба модема
When we speak, we transmit continuous sound waves (fig.1), or analog signals, that form what we call ‘voice’. Analog signals could be compared to a steady stream of water coming out of a garden hose. These signals repeat a certain number of times over a certain time period- their frequency, which is measured in cycles per second, or hertz. Sometimes our voice sound high (composed of high-frequency sound waves) and sometimes our voices sound low (composed of low-frequency waves). Analog signals can also differ in amplitude, or loudness; a soft voice is at low amplitude. Telephone lines are currently an analog communications medium.
C ontinuous, or analog signal
Digital signals, in contrast, can be compared to the short bursts of water that shoot out of a timed garden sprinkler. These signals are discontinuous (discrete) pulses over a transmission medium (fig.2). Computers communicate with each other in streams of binary digits (bits) transmitted in patterns of digital signals- a series of on and off electrical pulses. For data to travel from one computer to another across the phone lines, the sending computer’s digital data must be converted into analog across the phone lines, the sending computer’s digital data must be converted into analog form and then recovered into digital form at the receiving end - modulation and demodulation.
Discontinuous, or digital signals
Modulation refers to the process of converting digital signals into analog form so that data can be sent over the phone lines. Demodulation is the process of converting the analog signals back into digital form so that the receiving computer can process them. The hardware that performs modulation and demodulation is called a modem (modulate/demodulate). The sending computer must be connected to a modem that modulates the transmitted data, while the receiving computer must be connected to a modem to demodulate the data. Both modems are also connected to the telephone lines.
5. Retell the text.
Text 2
Telephone lines
Words and word combinations:
Unsheathed copper wires – не обшитые медные провода
to string – вешать
to secure – защищать
uninsulated – неизолированный
electromagnetic interference – электромагнитные помехи
twisted – скрученный
garbled – искаженный
susceptible – восприимчивый, чувствительный
digital signals – цифровые сигналы
an amplifier – усилитель
repeaters – трансляционный усилитель
more advanced media – более передовые средства
The earliest type of telephone line was referred to as open wire – unsheathed copper wires strung on the telephone poles and secured by glass insulators. Because it was uninsulated, this type of telephone line was highly susceptible to electromagnetic interference; the wires had to be spaced about 12 inches apart to minimize the problem. Although open wire can still be found in a few places, it has almost entirely been replaced with cable and other types of communications media. Cable is insulated wire. Insulated pairs of wires twisted around each other- called twisted-pair cable- can be packed into bundles of a thousand or more pairs. These wide-diameter cables are commonly used as telephone lines today and are often found in large buildings and under city streets. Even through this type of line is a major improvement over open wire, it still has many limitations. Twisted-pair cable is susceptible to a variety of types of electrical interference (noise), which limits the practical distance that data can be transmitted without being garbled. (To be received intact, digital signals must be ‘refreshed’ every one to two miles through the use of an amplifier and related circuits, which together are called repeaters. Although repeaters do increase the signal strength, which tends to weaken over long distances, they can be very expensive.) twisted-pair cable has been used for years for voice and data transmission; however, never, more advanced media are replacing it.